a buoy attached to a cable when slipping an anchor.
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society > travel > travel by water > vessel, ship, or boat > buoys, marks, or lighthouses > [noun] > buoy > buoy attached when slipping anchor
slip-buoy1798
1798 R. W. Miller in Ld. Nelson Dispatches & Lett. (1846) VII. p. clviii We hove up to our best bower..and got a slip buoy on the end of the sheet cable.